Justification:
This species is listed as Endangered because area of occupancy is probably less than 500 sq km, wild populations exist at no more than 5 locations, and (despite helpful conservation actions) habitat quality/quantity and the species' distribution and abundance are subject to ongoing declines from continuing threats. Long-term viability of populations introduced into refugia is uncertain, so these are not included in the number of locations.

Historically, this species was widely distributed in the Bonneville Basin, including streams near Salt Lake City, ponds and swamps around the Great Salt Lake, Utah Lake, Beaver River, Parowan Creek, Clear Creek, Provo River, Snake Valley, and elsewhere (Sigler and Sigler 1996).

Currently, only five wild populations of Least Chub remain (USFWS 2010). Three populations are in Snake Valley in Utah's West Desert. They include the Leland Harris Spring Complex, Gandy Salt Marsh, and Bishop Spring Complex. The remaining two wild populations are located on the eastern border of the native range near the Wasatch Front. Those populations in the Mills Valley and Clear Lake are in the Sevier River drainage. A functionally extirpated site exists at Mona Springs in the Utah Lake drainage.

In an attempt to create refuge (an artificial place of protection for a species) populations and reestablish wild populations, 19 introductions of Least Chub to new locations rangewide were attempted between 1979 and 2008 (see USFWS 2010). Of these, two sites are currently stable and secure (one has persisted for 3 years and another for 1 year), seven introductions failed, and three are not secure. The long-term success of seven of the transplants is currently unknown.

Total adult population size is unknown. Individual populations are small.

Decline was first noted in the 1940s and 1950s, continued through at least the 1980s. Recent recovery efforts have resulted in an increasing trend in abundance and in the number of populations (Muck 1999). However, data from the 1990s suggested that Least Chub numbers may have been declining within the Gandy Salt Marsh (Perkins et al. 1998).

Historically this species occurred in slow rivers, clear creeks, springs, ponds, and marshes (Sigler and Sigler 1996, Page and Burr 2011). Now it is basically an alkaline spring inhabitant. Typically it is found in moderate-dense submergent and emergent vegetation, at depths of 10-90 centimetres, over bottoms of clay, muck, mud, and peat (Sigler and Miller 1963, Lee et al. 1980).

USFWS (2010) found that listing the Least Chub as threatened or endangered is warranted due to the effects of livestock grazing and water withdrawal and diversions on the species and its habitat. Although the agencies have worked to protect Least Chub habitat with grazing enclosures where possible and grazing management plans in some areas, livestock-grazing-related impacts are still observed at most Least Chub sites. There is substantial evidence showing the negative effect of historical groundwater withdrawal on Least Chub. While uncertainty exists on the magnitude of effects to the Least Chub from proposed large-scale groundwater pumping, concern regarding the remaining five extant, wild populations is sufficient to indicate that the species is at risk of extinction in the foreseeable future, especially when combined with the threat of drought.

USFWS (2010) found also that listing the Least Chub as threatened or endangered is warranted due to the continuing threat of nonnative species, particularly Mosquitofish, for which there is no known means of control. Several significant efforts have been made to remove Mosquitofish from Least Chub habitats, without success. The wild least chub population at Mona Springs is functionally extirpated due to Mosquitofish, and nonnative fish are present at two of the five remaining viable populations.

USFWS (2010) found also that the Least Chub is at risk of extinction in the foreseeable future due to inadequacy of existing regulations to regulate groundwater withdrawals and ameliorate their effects on Least Chub habitat.

Further, USFWS (2010) found that the Least Chub is at risk of extinction in the foreseeable future because of the cumulative effects of drought, current and future groundwater withdrawal, and climate change on the remaining naturally occurring populations in Snake Valley.